1. Field of the Invention
The present invention relates to a method for driving a solenoid valve serving as a fluid control device favorably, a solenoid valve driving apparatus, and a combustion apparatus including the solenoid valve driving apparatus.
2. Description of the Related Art
A solenoid valve is used to switch a fuel supply in a combustion apparatus ON and OFF, for example. Japanese Patent Application Publication No. S58-65384 describes an example of an apparatus for driving this type of solenoid valve.
The apparatus described in this publication drives a solenoid valve using a PWM (Pulse Width Modulation) system, includes a switching element for switching application of a direct current voltage to a solenoid of the solenoid valve ON and OFF, and uses a PWM signal as a control signal for operating the switching element. When the solenoid valve is caused to perform a valve opening operation, a signal having a duty ratio of 100% is input into the switching element. After the solenoid valve has entered an open state, a PWM signal having a lower duty ratio than the signal used during the valve opening operation is input into the switching element as means for maintaining this state. According to this constitution, an amount of power supplied to the solenoid can be increased during the valve opening operation of the solenoid valve, and therefore the valve opening operation can be performed reliably. While maintaining the open state thereafter, the amount of power supplied to the solenoid can be suppressed, and therefore a reduction in energy consumption can be achieved.
With the conventional technique described above, however, the following problems may occur.
During the valve opening operation of the solenoid valve, a voltage is simply applied to the solenoid at a duty ratio of 100%, and therefore, although the solenoid can be supplied with a large amount of power, a comparatively loud noise may be generated as the valve opens. This noise is generated when a valve body of the solenoid valve and a plunger that supports the valve body are moved sharply by a large electromagnetic force from the solenoid such that a part of the plunger collides with a casing of the solenoid valve, for example. It is desirable to suppress this noise to the greatest extent possible.
As means for preventing this noise, the power supplied to the solenoid may be reduced so that the valve body does not move sharply. However, when this means is employed alone, it may be difficult to execute the valve opening operation appropriately. More specifically, for example, when a voltage of a direct current power supply connected to the solenoid valve becomes lower than a normal voltage for any reason, the power required for the valve opening operation may not be supplied to the solenoid valve, making it difficult to execute the valve opening operation appropriately. Further, when a combustion apparatus including the solenoid valve is driven to perform combustion for an extended period of time such that a temperature of the solenoid valve rises, an electric resistance of the solenoid increases, leading to a reduction in a current flowing to the solenoid. Hence, it may also be difficult to execute the valve opening operation when a combustion operation is resumed in the combustion apparatus after stopping the combustion apparatus following an extended combustion operation.
Furthermore, with the conventional technique described above, during a period in which the solenoid valve is maintained in the open state, a voltage is simply applied to the solenoid at a lower duty ratio than that of the valve opening operation such that the amount of power supplied to the solenoid is continuously suppressed. Hence, when a momentary power failure (for example, a power failure that is too short for an operation-controlling microcomputer to be reset) occurs such that the solenoid valve enters a closed state, the amount of power supplied to the solenoid following resumption of the power supply is small, and it may therefore be impossible to return the solenoid valve to the open state appropriately. The solenoid valve may be switched from the open state to the closed state unintentionally due to large pressure variation in a pipe passage to which the solenoid valve is attached, but with the conventional technique described above, it may likewise be impossible to return the solenoid valve to the open state in such a case. As a result, the solenoid valve remains in the closed state when it should be in the open state, and this situation is undesirable.